This invention relates to pressure filters, and more particularly, is concerned with tube pressure filters incorporating a novel filter element and with a method of fitting said filter element on a tube pressure filter.
In recent years there has been developed a type of pressure filter, conveniently called a tube pressure filter, which is capable of operating at high pressures, for example pressures in excess of 1000 lbs/in.sup.2, and which can be used to separate particulate solids and liquids which are not separable to the same extent by conventional plate filter presses. Various embodiments of tube pressure filter have been described, see for example British patent specification Nos. 907,485; 1,194,676 and 1,271,494, but in general a tube pressure filter essentially comprises (a) a pair of generally coaxial inner and outer tubular bodies arranged one within the other and adapted to be supported in a generally upright position, (b) an impermeable elastic sleeve disposed within and secured to the outer tubular body, (c) a filter element disposed around and supported by the inner tubular body, (d) first outlet means whereby filtrate which has passed through the filter element can be discharged from the tube pressure filter, and (e) second outlet means whereby solid retained on said filter element can be discharged from the tube pressure filter, the arrangement being such that in a first operating condition of the tube pressure filter said second outlet means is closed and the tubular bodies co-operate with each other to define a closed annular chamber which is divided into generally coaxial and non-intercommunicating inner and outer compartments, the inner compartment having an inlet for feed material (comprising a mixture of a liquid and a particulate solid) and the outer compartment having an inlet for a hydraulic fluid under pressure, and in a second operating condition of the tube pressure filter said second outlet means is open to enable the particulate solid to be discharged from the inner compartment.
In one kind of tube pressure filter, the tubular bodies are movable relative to one another and the inner tubular body comprises a cylindrical central section, around which cylindrical central section there is disposed the filter element, and upper and lower end sections secured one to each end of the cylindrical central section, each of which end sections includes a portion (which generally co-operates with the adjacent portion of the outer tubular body to form the closed annular chamber when said tubular bodies are in their first operating condition) of greater diameter than said cylindrical central section; and the tube pressure filter further includes means for displacing the tubular bodies axially relative to one another whereby the tube pressure filter can be placed in either one of its first and second operating conditions. Embodiments of this kind of tube pressure filter are disclosed, for example, in U.S. Pat. No. 3,756,142. Hereinafter, such a tube pressure filter will be referred to as "a tube pressure filter of the kind set forth". Heretofore in tube pressure filters of the kind set forth the filter element has comprised a filter cloth sleeve which fits closely around the inner tubular body usually with a backing cloth and wire mesh sleeve being disposed between the filter cloth sleeve and the inner tubular body. Generally, the filter cloth sleeve has been woven and tailored to a shape and size enabling it to be fitted directly as a replacement for a filter cloth sleeve which has been damaged or whose interstices have been blocked by solid particles. When these known filter cloth sleeves are fitted on the inner tubular body the warp and weft filaments thereof run longitudinally and circumferentially of the inner tubular body and it is not possible to stretch the filter cloth sleeve laterally by more than a very small amount, say 5% at the most. With some forms of tube pressure filter of the kind set forth the fitting (sometimes known as the dressing) of the filter element is rendered more difficult by the fact that the end sections of the inner tubular body have a significantly larger diameter than the cylindrical central section which supports the filter cloth sleeve, so that in order to fit the filter cloth sleeve at least one of the end sections of the inner tubular body has to be removed, with a consequential expenditure of time and effort.